A driving force of the engine of a wheel loader is distributed to dual power transmission paths for traveling and working. In other words, the driving force of the engine is supplied to a traveling power train and a hydraulic pump for a work machine through a PTO shaft and distributed to traveling horsepower and working horsepower.
An input clutch (modulation clutch) is disposed between the engine and the transmission on the power transmission path of the traveling power train.
The input clutch is disposed to adjust the power transmitted to the traveling power train and to increase the power transmitted to a work machine via the hydraulic pump for the work machine or to prevent the vehicle from slipping depending on work conditions.
In a case where a wheel loader performs an excavation work by digging into the ground, it is necessary to decelerate the vehicle while depressing an accelerator pedal to keep the engine at a high rotation speed. Therefore, a brake pedal is depressed to decelerate the vehicle by operating the brakes, and also the clutch pressure of an input clutch is decreased to enhance a braking effect and to decrease the engine driving force transmitted to the traveling power train, thereby distributing a larger driving force to the hydraulic pump for the work machine.
Japanese Patent Application Laid-Open No. H01-186440 discloses an invention which is related to a general vehicle and aims to prevent engine stall at the time of braking by disengaging the clutch by supplying a brake operating oil to a cylinder for operating the clutch through piping when the brake pedal is depressed.
Japanese Patent Application Laid-Open No. H03-248929 discloses an invention which is related to a forklift, and aims to perform an inching operation by means of the brake pedal by operating a master cylinder for the brake pedal through a piston rod when the brake pedal is depressed and by pushing or pulling a cable in association with the operation of the brake pedal to operate an inching control valve.
To control both of the brake device and the input clutch depending on a depressed amount of the brake pedal, it is necessary to adjust a relationship between a clutch pressure of an input clutch and a braking force (brake pressure) generated by the brake device to a desired relationship as shown in, for example, FIG. 3. It is desired that the adjustment can be made easily by a simple structure.
But, according to the invention described in JP H01-186440, it is configured to control the clutch pressure of the input clutch by the brake operating oil. Therefore, the relationship between the brake's braking force and the clutch pressure is determined uniquely depending on the designs of hydraulic oil pipe and hydraulic equipment, and the relationship cannot be changed easily. The clutch pressure is varied by an influence of a temperature of the brake operating oil or the like, and stable characteristics cannot be obtained.
Further, JP H01-186440 is directed to a general automobile and is essentially different from the present invention, which is directed to a work vehicle such as a wheel loader in that an engine driving force is distributed to dual power transmission paths for traveling and working.
The invention described in JP H03-248929 is directed to the control of an inching clutch, which is disposed within the transmission and is essentially different from the present invention which controls the input clutch disposed between the engine and the transmission.
The invention described in JP H03-248929 is configured to independently operate the braking master cylinder and the inching control valve, so that the structure becomes complex and the adjustment cannot be made easily.
The present invention has been made in view of the above circumstances and has an object to overcome the above problems by facilitating the adjustment of the relationship between the clutch pressure of the input clutch and the braking force (brake pressure) generated by the brake device to a desired relationship by a simple structure.